This invention relates to spacecraft attitude control systems, and more particularly to such systems using sensed roll applied to a compensator, which uses orbit-rate bandpass filtering to reduce roll and yaw.
Satellites or spacecraft are in widespread use as platforms for communications systems and sensors. Advances in spacecraft design have resulted in spacecraft which carry progressively larger solar panels to provide power for increasingly more sophisticated electronic systems and more powerful transmitters, and which also carry progressively larger antenna structures for increasingly complex beamforming applications. These more advanced systems often require high pointing accuracy for their sensors and or antennas, so 3-axis stabilization of the satellite body is required.
As the satellite structures become larger, the environmental torques exerted on the spacecraft, as by solar wind, light or photon pressure, magnetic fields, particles and the like, tend to become greater. These larger disturbance torques tend to cause greater attitude perturbations than those experienced with smaller spacecraft, which tends to degrade the pointing accuracy.
An article entitled THE SYNTHESIS OF REGULATOR LOGIC USING STATE-VARIABLE CONCEPTS, by Bryson and Luenberger, appeared at pp 1803-1811 of the PROCEEDINGS OF THE IEEE Vol. 58, No. 11, November, 1970, and constitutes a primer for automatic control systems. As described at pages 1805 and 1806, an auxiliary dynamic system is termed a "filter" which is used to "estimate . . . state variables" of a system. This filter, used with state-variable feedback, constitutes a "compensator". A special filter design termed "observer" may be used to estimate x(t), in a system defined by x=Fx+Gu, when the system input u(t), the system output z(t)=Hx(t), initial conditions x(t.sub.0), and F and G are known. An open-loop estimate (x) of x(t) may be made by applying u(t) to the system, ##EQU1## but the open-loop estimate, in practice, quickly becomes inaccurate because F, G, u(t) and x.sub.o (t) are never accurately known. This difficulty is overcome by a closed-loop estimator termed an "observer", in which the closed-loop error z-Hx is fed back into equation (1) to produce ##EQU2##
These principles have been applied to satellite control, as described, for example, in U.S. Pat. No. 3,813,067, issued May 28, 1974 in the name of Mork. The Mork attitude control system does not require knowledge of yaw angle and therefore does not need sun-sensors or gyroscopes. It is advantageous to operate, if possible, without sun sensors and or gyroscopes, for they tend to be less reliable than some other types of sensors. In the Mork arrangement, a gimballed momentum source or wheel is oriented for momentum, principally along the pitch axis, but may be moved on its gimbals, whereby measurements of roll and yaw gimbal angles (.gamma..sub.x, .gamma..sub.y, respectively) give a basis for determining measured momentum values (H.sub..omega.x, H.sub..omega.z) along these axes. The yaw and roll axes are stabilized by the gyroscopic stiffness along the pitch axis. The yaw accuracy of the Mork system is improved by torque compensation logic, which models the expected disturbances about the roll and yaw axes, and which provides correction signals to the yaw and roll gimbal controls.
Mork's torque compensation logic receives as inputs a roll error signal (.phi.) from one or more roll sensors, and the roll and yaw gimbal angle (.gamma..sub.x, .gamma..sub.z) signals. The roll error signal .phi. and the roll gimbal signal .gamma..sub.x determine the measured momentum. The logic produces yaw gimbal angle command signal .gamma..sub.zc. The yaw gimbal is coupled to receive .gamma..sub.zc from the logic, and the gimbal angles are controlled by a gimbal motor to provide 3-axis stabilization by the reaction torques caused by the changing gimbal angles. Mork's logic multiplies .gamma..sub.x, the roll gimbal angle, by a constant .omega..sub.o, and adds to the result a compensation signal derived from signals representative of the momentum control torque about the roll axis, including yaw magnet drive, to thereby produce a first intermediate signal. Also, roll error signal .phi. is multiplied by a constant K.sub.2, to generate a product K.sub.2 .phi. which is integrated to produce a second intermediate signal. The second intermediate signal is added to the product of a constant K.sub.1 multiplied by .phi. to produce a third intermediate signal, which is added to the integral of the first intermediate signal to produce the desired yaw gimbal command signal for application to the yaw gimbal.
U.S. Pat. No. 4,521,855, issued June 4, 1985 in the name of Lehner et al., describes a satellite 3-axis control system with momentum bias along the pitch axis. The yaw accuracy of the Lehner et al. system is improved by an apparatus for estimating and directly correcting yaw error, thereby avoiding the need for a sun sensor. FIG. 1 is a simplified block diagram of the Lehner arrangement. An earth sensor assembly (ESA) 10 measures the roll error angle and produces a roll error signal as well as a pitch error signal. An electronic logic arrangement (estimator) 12 is coupled to roll sensor 10 to estimate the yaw error, using the roll error and the measured momentum about the yaw axis as inputs. The roll signals, and the estimated yaw error, and disturbance torque signals produced by estimator 12 are summed together in a summer 14 to produce torquer drive signals. The torquer drive signals are applied to a magnetic torquer drive unit illustrated as a block 16, which applies appropriate signals to one or more coils of a set of torquer coils, illustrated together as a coil 18. In order to distinguish disturbance torques acting on the spacecraft from the effects of the magnetic torquing commanded by the estimator, the estimator must be provided with information relating to the torque contributed by the torquer. A feedback path illustrated as 20 is coupled from magnetic torquer drive 16 to estimator 12, and couples magnetic torquer drive signals to the estimator, where the torquer contribution may be subtracted from the overall disturbances to produce the environmental disturbance torques.
It has been discovered that the magnetic torques generated by the magnetic torquers may differ considerably from the commanded value, because of variations in the earth's magnetic field. Since the magnetic torques are subtracted from the total disturbing torques to produce environmental torques, this difference between commanded and actual torque may result in very large inaccuracies in the estimated disturbance torques, resulting in substantial errors in the yaw pointing accuracy. Inaccuracies in the magnetic coil current, which may be due to source voltage or temperature effects, may also affect the actual torque.
An improved roll-yaw control system is desired.